Problem with sim cards, SIM1 & 2

I installed latest version of MIUI on my device (Jiayu G4 dual sim). The problem is I can't activate 3g on sim2, I almost drained the battery trying different settings and I gave up. SIM 1 is microsim (voice only Orange), SIM 2 is normal size sim (3g only Digi Mobil). Whatever I do, I can't activate 3g on Digi, when I manually search for network it gives me only 2g networks and SIM 1 is using dual mode, I want to set it up GSM only. What can I do? Please help me, I love MIUI and I want to stay with it. On other roms there aren't problems.